Is sugar water good for plants? This is a very interesting question and one that leads to all kinds of fun facts about plants.
Even more interesting is how this myth plays out on the internet. In most cases, popular gardening sites promote the myth, but in this case they tell you NOT to use sugar water for plants, except in special cases. They then give a variety of reasons why sugar water is bad for plants, but these explanations are mostly wrong. They are creating new myths with their explanations!
Many of these writers ignore basic plant biology or are unaware of the correct facts. I’m going to try and sort this thing out for you.
Should You Use Sugar Water For Plants?
The right amount of sugar will be beneficial to plants. The problem for the gardener is to know what the “right” amount is. Too little and it is a waste of time. Too much and you harm your plants.
A well grown plant does just fine without a sugar source. I think it is better to focus on growing plants correctly, rather than experimenting with sugar amounts. However, I might start adding sugar to cut flowers.
If you would like to try sugar, read the following sections.
What is Sugar?
The term “sugar” can be misleading. To the layperson, sugar is the white stuff we sprinkle on our cereal and its chemical name is sucrose.
The term sugar is also used to describe a family of compounds that are carbohydrates (contain carbon, hydrogen and oxygen) and their names often end in “ose”: glucose, dextrose, fructose, maltose, lactose, etc. Some of these sugars are simple sugars that contain one sugar subunit and are called monosaccharides. When two of these join together they are called disaccharides and table sugar is an example of a disaccharide. Sucrose is a combination of glucose and fructose.
You probably know that plants use photosynthesis to make “sugar”, but which sugar? To understand this better have a look at my book Plant Science for Gardeners. Plants use photosynthesis to produce glucose and then convert glucose to sucrose because it is more soluble in water and easier to transport around the plant. Sucrose is also a more efficient way to store energy – it is a better type of battery than other types of sugars.
What is Osmotic Pressure?
Some chemicals are able to dissolve in water and when this happens it changes the physical properties of water. One of these properties is the osmotic pressure. Pure water has a low pressure of zero and dissolving chemicals in it increases the osmotic pressure. A sugar solution has a higher osmotic pressure than water. A concentrated solution has a higher osmotic pressure than a dilute solution.
Cells have special membrane layers around them which are semi-permeable. In simple terms this means that water can move freely through the membrane but many other compounds can’t. Water always flows from an area of low pressure to one of high pressure. So if the osmotic pressure is higher inside the cell than outside the cell, water will flow into the cell. Plants take full advantage of this in their roots. The leaves make sugars and pass them down to the roots. This makes the water inside the roots sweeter and increases its osmotic pressure. This causes water to flow from outside the root (area of low osmotic pressure) to inside the root.
Is Sugar Water Good for Plants?
There is no single answer which is one reason many discussion on this topic fail. To answer the question you have to first define the concentration of sugar in the water.
At very low concentrations, the sugar will have limited impact on anything. It might help the plant a bit but it won’t harm it.
High concentrations affect plants quite differently. High sugar solutions near roots will have a higher osmotic pressure than inside the root. This causes water to flow out of the root instead of in, dehydrating the plant and killing it.
This is no different than adding too much synthetic fertilizer to soil. The fertilizer dissolves in the soil water, increasing the concentration near roots, which creates a higher osmotic pressure outside the plant. Water then flows out of the roots and the plant dies.
Many online sites provide a formula for making sugar water but none of the ones I reviewed specified how much to add to the soil. Without that information you have no idea if it is too much sugar. Suggested mixtures range from 0.5 tabs/quart to 1/4 cup/4 cups (8 to 60 ml/l).
It is perfectly clear from our understanding of osmotic pressure that a high amount will kill plants. So the question we need to ask, and the one I will discuss in the rest of this post; is sugar water at lower doses beneficial to plants?
Do Plant Roots Absorb Sugar?
I will use the term “sugar” in the rest of this post to mean table sugar, namely sucrose.
One of the main arguments against adding sugar to plants is the claim that “plants do NOT absorb sugar through their roots”. But that is not true. Plants can absorb sucrose through the roots.
Plants make sucrose and use it as an energy source. If they can absorb it through their roots adding it to soil should benefit the plant, after all it does not mater if the sucrose came from the grocery store or was made in the leaves of a plant.
Plants expel sugar through their roots in the rhizosphere to attract microbes and they can reabsorb any excess sugar.
Sugar Activates Bacteria
Sugar is a great energy source for bacteria in the soil. They gobble it up and start reproducing. All this microbial activity is claimed to be good for plants and it is in some ways, but it also harms plants.
Higher microbe activity means that more nutrients are extracted from the soil and as the bacteria die these nutrients become available to plants. This is the positive aspect.
However, in order for microbes to grow they also need oxygen and nitrogen. Both of these are removed from soil decreasing the amount available to plants. I am not sure if the decrease in oxygen is significant enough to affect plants, but a lack of nitrogen can certainly be a problem.
What happens when the sugar is used up? The above chart shows what happens to the bacteria population when sugar is added to soil. There is a rapid increase in population until the bacteria use up all of the sugar. Suddenly, many bacteria die from lack of food. For a while the living can consume the dying, but in fairly short order the population is back to the starting point.
Is this a benefit to plants? There is some short term benefit from extra nutrients and some short term harm do to a lack of nitrogen. The net effect is that there is not much benefit to plants unless the sugar is constantly added to maintain a higher population and extra nitrogen is available.
Sugar Affects Flowering
Sugar is used by plants as a signaling molecule that regulates a variety of internal biological processes and one that is of direct interest to gardeners is flowering. When a plant enters the flowering phase there is an increase in sugar level near the bud meristem. The extra sugar signals the bud to form and eventually flower.
However, this does not mean that adding sugar to a plant will make it flower. Flowering is a complex process that requires several triggers to be in place and the sugar level is just one of these triggers. Day length is another trigger and it also has to be right before bud development takes place.
The other thing that is important is the actual sugar level. Lower amounts of sugar initiate the growth of buds, but high levels inhibit the growth of buds.
It is hard to see how a gardener can water with sugar and get the “amount” right for flowering. Remember it is the internal concentration that matters – not the amount you apply to soil. Sugar may be useful in plant production where a nursery controls the sugar level and timing of the application.
Does Sugar Affect Crop Yield?
A study in 2013 looked at sugar application to corn and soybeans and found no increase in yield. Another study sprayed various types of sugar on soybean crops and found no yield benefit. It compared 4 different sugar sources including molasses which many gardeners claim is superior to other sources of sugar. The sources studied were:
- Granulated cane sugar (100% sucrose)
- High fructose corn syrup (11 g glucose/fructose per 30 mL)
- Molasses (28 g sucrose per 30 mL)
- Blackstrap molasses (26 g sucrose per 30 mL)
In this study molasses was not better than the others, nor was it better than water.
A simple experiment growing radish seeds with and without sugar found they grew better with no sugar.
Using Sugar on Cut Flowers
Commercial cut flowers are harvested before they are fully developed and to develop properly they need carbohydrates. In uncut flowers these carbohydrates are supplied by the rest of the plant, but that can’t happen once they are cut. Sugar added to the water can provide the needed energy to fully develop the bloom and most flowers benefit from the addition of 2% sugar. Some, like gladiolus benefit from higher amounts (4-6%) and yet other plants (zinnias and coralbells) sustain damage at anything over 1%. Chrysanthemums and China asters, do better with no sugar added.
A 1% sugar solution contains 10 g/l (2 tsp/quart).
A sugar solution will encourage the growth of bacteria so you should also add a biocide to any sugar solution (50 ppm household bleach, 1 ml/l).
Sugar Water for Christmas Trees?
It won’t keep a Christmas tree fresh longer than just plain water.
Some Common Claims About Sugar Water
In this section I will list some claims I found on common gardening blogs and discuss their accuracy.
Sugar is a Good Fertilizer
Sugar contains carbon, oxygen and hydrogen. It contains none of the nutrients plants get from soil and is therefore not a fertilizer.
Sugar Provides Plants with Nutrients
Based on the previous statement you might think that this is false, but it is at least partially true. Sugar does not provide nutrients directly, but it can provide them indirectly. Microbes are better at extracting nutrients from soil than plants, especially for things like phosphate. As microbes die they make these nutrients available to plants. Microbe activity also lowers soil pH which makes some of the metallic nutrients, like iron, more available to plants.
Brown Sugar is Better Than White Sugar
This is wrong for two reasons.
First of all, plants benefit from sucrose. The sucrose in both white and brown sugar is identical.
Secondly, most commercial brown sugar is made by taking white sugar and adding some molasses to it. Brown sugar is essentially white sugar colored with molasses. Unrefined brown sugar is sugar that has been refined to a lessor degree, but it is still mostly sucrose.
Heavy Metals are More Readily Available and Will Harm Plants
The microbial growth mentioned above results in a lower pH which makes certain nutrients, including heavy metals, more available. The amount of these in soil is usually so small that this will not have a significant effect in most soils.
Sugar Hydrates Plants in a Drought
I have no idea where this idea came from but it might be a super-extrapolation of the way sugar controls things in plants. Sugar does activate a number of metabolic functions inside a plant including the activation of stress-inducible genes. But there is a big difference between sugar levels in a plant and sugar added to soil.
The level of soil water during drought conditions is very low. As levels drop the sugar in the soil will become more concentrated which causes an increase in osmotic pressure. That won’t hydrate a plant – it does the opposite. Using sugar water on a drought stressed plant might help because you are adding water, but it won’t be due to the sugar.
Alternate Pure Water with Sugar Water
Some of the advice online suggests watering with sugar water and then using pure water in subsequent waterings. It is claimed that, “the pure water will dilute and keep the amount of sugar to a safe level”. But if the original sugar water was at safe level, and bacteria consume excess sugar, why would you need to dilute it? None of this makes any sense.
If sugar water, at prescribed concentrations, is good for plants then you should be able to use it with every watering.
Adding Sugar to Soil Makes Tomatoes Sweeter
Tomato flavor is determined by the environment and genetics. Adding sugar to the soil will not make them sweeter.
Sugar Water Controls Weeds
The claim is that sugar encourages roots to seek nitrogen in soil which depletes the nitrogen level and in turn slows down plant growth in weeds.
Assume for a minute that sugar does encourage roots to find more nitrogen. Why would sugar only have this effect on the plants we want to grow and not on the weeds? This is a mistake in logic I see all the time in gardening circles. For some reason our precious plants have a different biology than weeds! I have news for you – weeds DO NOT KNOW they are weeds.
Roots are always looking for nitrogen and it is the nutrient that is usually in short supply. I see see no reason why sugar would accelerate this process? In fact, sugar may actually stimulate nitrogen-fixing bacteria which would increase nitrogen levels.
Sugar Water is Good for Dying Plants
The claim is that sugar water is “wonderful for boosting dying plants but it should not be used for every day watering”.
So sugar helps a dying plant but not a healthy plant? You’ve got to love such illogical claims that don’t even try to explain why they might be true – they just are!
Plants Use a Different Sugar
It is claimed that “plants do not metabolize sugar as humans do and the sugars they produce (glucose) have a different make-up to the polysaccharides of our store-bought sugar”.
Plants and animals do metabolize things differently but when it comes to converting sugar to energy, they both use the same set of reactions known as “glycolysis”, the splitting of glucose. It is even used by microbes.
Store bought sugar is sucrose, a disaccharide, not a polysaccharide (e.g. starch).